1. Field of the Invention
The present invention relates to a sensor magnet device, a gear mechanism and a speed reducing electric motor.
2. Description of Related Art
The present invention relates to a sensor magnet device, a gear mechanism and a speed reducing electric motor.
A known speed reducing electric motor includes a speed reducing mechanism, which is formed as a unit that includes a worm speed reducing mechanism connected to a motor main body. An annular magnet, which serves as a sensing subject of a rotational position sensor (magnetic sensor), is coaxially fixed to a worm wheel of the worm speed reducing mechanism, which rotates together with an output shaft. Such a speed reducing electric motor is disclosed in, for example, Japanese Unexamined Patent Publication No. 2005-94821. According to the technique disclosed in this publication, the magnet is rotated together with the output shaft. A magnetic pole change, which is made by a characteristic magnetic pole pattern of the magnet, is sensed with a Hall IC to determine a rotational position of the output shaft.
However, the magnet of the above technique is merely configured into a simple annular body, so that there is still a need for improving the magnet to sense a greater amount of information about a rotational position (rotational state) of the output shaft to increase a sensing accuracy of the rotational position sensor.
Furthermore, the magnet of the above technique is fixed to the worm wheel by bonding the magnet to the worm wheel with a bonding agent, by snap-fitting claws of the worm wheel to a peripheral surface of the magnet or by welding a portion of the worm wheel upon installation of the magnet to the worm wheel to limit unintentional removal of the magnet. The first two methods, i.e., the bonding and the snap-fitting have the disadvantages discussed in Japanese Unexamined Patent Publication No. 2005-94821. Furthermore, heat generated at the time of welding of the portion of the worm wheel may possibly cause a deformation of the worm wheel and/or the magnet (particularly in a case of a bond magnet). The deformation of the gear and/or the magnet may possibly cause an error in a measurement of the rotational position sensor to deteriorate the sensing accuracy of the rotational position sensor.